A Day at the (Ancient) Beach EarthCache
EggSilent4: Thanks to all who log this one correctly per the requirements.
A Day at the (Ancient) Beach
Please note Use of geocaching.com services is subject to the terms and conditions
in our disclaimer.
You are about to embark on a 7.5 mile hike along well maintained trails within the Cane Creek Canyon Nature Preserve to view an ancient barrier island beachfront. There is no fee for the use of this area but they do ask that you sign in. They also have very nice, heavy duty maps of the area near the sign in book for you to use and return. While the trails are well marked, they can be confusing at times and a map will come in handy for reference.
Lets begin with a bit of history for the land we now know as North America. During the Mississippian Period (340 M.Y.A) all of the Earth's continents were in the early stages of a collision that would lead to the formation of Pangaea, the ancient supercontinent formed near the end of the Paleozoic Era. Ancient Alabama lay well south of the Earth's equator at the time, at the southern margin of an ancestral continent that geologists call Laurasia. A huge collection of what are today the southern continents (given the name Gondwana) was being driven northward by tectonic forces. Signs of this movement appear in Colbert County's rock record as a shift from limestone deposition in the clear seas that separated the northern and southern land masses (at the bottom of the rock column) to muds and sands entering the sea farther up in the rock record. This mud and sand is now preserved in the layers of the Pride Mountain Formation. Later still in time, the sandy sediments were re-worked by waves and currents to form the Hartselle Sandstone.
As you hike from the far end of the Underbluff Trail, you are moving from a point offshore of the beach, into shallow, nearshore waters, right at the beach front itself (it's difficult to say whether above or below wave base) and then possibly into the back bay, shoreward of the islands. All of those plant fragments in muddy sandstone just before starting downhill might indicate the sediments came from the back bay area.Notice the changing of the rock, both in composition, texture, and in some cases, color along this trail. The rocks you are seeing along the cliff face changed both because your position changed in relation to the ancient beach, but also because, through time, a slightly different set of rocks was being deposited as the position of the beach moved because of changing sea level. Notice around the halfway point how the rocks are a very light, almost white, sandstone. That is the ancient beachline on the offshore portion of the barrier island. Look at the image below and imaginge walking from the ocean, up to the barrier island and over it into the back bay area.
We chose to start at the south end of the Underbluff Trail, at the upper end of Devil's Hollow, because by walking north you start offshore of the ancient beach front and work your way closer to the beach as you hike. The first sand bars you come to are thin, almost perfectly horizontal sand beds deposited in deeper ocean water. Higher up on the cliff face you see thicker sand beds with dipping marks showing high-energy water disturbed by wave action and currents.
Each line in the rock face has a meaning. The dipping marks are known as crossbeds. They represent water that was moving, as opposed to the fairly calm, deeper water represented by the thinner, more horizontal beds of sandstone.
At a spot about half way down the Underbluff Trail you reach a spot that may be the closest point to the ancient beach that is still preserved in the rock. The sand here is clean, white beach sand similar to what you might see on the beach at Gulf Shores today. The sand grains are tiny crystals of quartz that have been well-sorted to a similar size by the action of the pounding surf and currents along the ancient beach. On a sunny day, they sparkle like tiny diamonds in the cliff face.
By looking at the crossbedded sandstone from different angles you are able to see which way the ancient beach was facing—which direction was the land, and which direction lay the open ocean. The general dip to beach deposits is toward the ancient ocean basin, but these sand beds have been swept slightly to the right by longshore currents that have curved the beds somewhat in the general direction of the current flow along the ancient beach front. It is difficult to say if any sand dunes are preserved here. Actual beach dunes are rarely preserved in the geological record because they are so easily eroded by storms. This is probably the case here, as the upper part of the steepest and largest crossbeds have been carved away into a flat surface by storms at some later point in time.
Some of the most spectacular points of the trail are where you pass beneath towering sandstone overhangs where massive boulders have been undercut, and then fallen from the canyon wall. Canyons are shaped by this process of bouldering, which leaves a fresh, new cliff face for geology students to examine and interpret. In the science of geology "the present is the key to the past." Geologists learn to recognize ancient environments preserved in sedimentary rocks by studying modern environments in which rocks are forming. The Hartselle Sandstone is rich in these kinds of clues. One common feature in the sandstone at Cane Creek are ripple marks left in the sand beds just offshore of the ancient beach. Both wind and tidal flow can create ripples in sand beds in shallow water.
Fossils of shelled creatures that lived near the ancient beach are very rare in these rocks because limy shell material does not last long in sandstone bathed in acidic rainfall. However, fossilized fragments of plants that lived on the sandy islands and back barrier marshes are fairly common. This group was unusually good at spotting these small fossil wood fragments, both protruding from the cliff face as well as scattered in the rock rubble at the base of the cliff. Note to hikers: Please do not remove these fossil wood fragments from the canyon wall—leave them there for future hikers to ponder and enjoy far into the future.
At the north end of the Underbluff Trail you are forced to leave the shelf and descend to the canyon floor along the trail. (Noted as Trailhead 2 in the additional waypoints) The bluffs continue on, but the shelf path is blocked by huge sandstone boulders that have fallen from the cliff at this point.
Educational requirements: To claim a find for this earthcache please email via our profile link above the answers to the follow questions. Please include "GC3V1GQ" in the heading of your message to avoid confusion. "Found" logs without this information will be deleted.
1.What Period was this area believed to be under water and what was this ancient super continent called?
2. What do the dipping marks known as crossbeds represent?
3. Why are fossils rare in these rocks?
4. Take GPS elevation readings at the beginning, end and several points along the trail. Compare these elevations a explain your findings.
5. A photo posted with your log entry would make a nice addition to commemorate your visit. Although not a requirement, it would be appreciated.
The photos and portions of the text for this earthcache are the property of Dr. Jim Lacefield and used with his expressed permission for the educational benefit of this earthcache. There are several other photos in the gallery section if you would like to take a look at those as well.
Additional Hints
(Decrypt)
[In case of emergency dial 911
For directions or further information please contact
Cane Creek Canyon Nature Preserve
251 Loop Rd, Tuscumbia, AL 35674
(256) 381-6301]
Treasures
You'll collect a digital Treasure from one of these collections when you find and log this geocache:
Loading Treasures